A continuous galvanizing apparatus has such a structure as typically shown in FIG. 7. This continuous galvanizing apparatus comprises a pot 4 containing a molten zinc 3, a snout 2 immersed in a surface-layer portion of the molten zinc 3 to prevent the oxidation of a steel strip 1 introduced into the molten zinc 3, a sink roll 5 disposed in the molten zinc 3, a pair of support rolls 6, 6 positioned above the sink roll 5 in the molten zinc 3, and a gas-wiping nozzle 7 positioned slightly above the surface of the molten zinc 3. Without being applied an external driving force, the sink roll 5 is driven by contact with the running steel strip 1. One of the support rolls 6, 6 is a driving roll connected to an external motor (not shown), and the other is a driven roll. Non-driving-type support rolls, to which no external driving force is applied, may be used. The sink roll 5 and a pair of support rolls 6, 6 are fixed to a frame (not shown), and always immersed in the molten zinc 3 as an integral means.
The steel strip 1 enters into the molten zinc 3 through the snout 2, and its moving direction is changed by the sink roll 5. The steel strip 1 moving upward in the molten zinc 3 is sandwiched by a pair of support rolls 6, 6, to keep its pass line and prevent its warping and vibration. The gas-wiping nozzle 7 blows a high-speed gas onto the steel strip 1 exiting from the molten zinc 3. The thickness of the molten zinc attached to the steel strip 1 can be uniformly controlled by the gas pressure and blowing angle of a high-speed gas. Thus, a galvanized steel strip 1′ can be obtained.
Because the sink roll and the support roll used in the galvanizing pot are exposed to an extremely corrosive environment due to the molten metal, they have conventionally been made of iron materials having excellent corrosion resistance such as stainless steel, chromium-based, heat-resistant steel, etc. However, these rolls' surfaces are disadvantageously corroded and worn by immersion in the molten metal pot for a long period of time. Thus proposed as rolls for use in galvanizing pots are those having bodies made of ceramics having excellent corrosion resistance, heat resistance and wear resistance, which are brought into contact with steel strips.
JP5-195178A discloses a support roll rotating in a galvanizing pot in contact with a steel strip, which comprises a hollow steel roll, and an oxide- or carbide-based ceramic coating thermally sprayed on the hollow steel roll, the ceramic coating being dulled to have a surface roughness Ra of 1.0-30 μm. With the thermally sprayed ceramic coating having a surface roughness Ra of 1.0-30 μm, friction increases between the roll and a steel strip, thereby preventing the roll from insufficiently rotating and the steel strip from being damaged due to the insufficient roll rotation. However, because a ceramic is thermally sprayed on an iron roll matrix, cracking occurs in the ceramic coating due to the difference in a thermal expansion coefficient between the matrix and the ceramic coating. As a result, the roll is disadvantageously corroded from cracks to be drastically worn.
The drastically worn roll cannot keep circularity, thereby causing vibration in the roll and the steel strip. It is thus impossible to obtain a steel strip with a uniform-thickness plating. Accordingly, a plating operation should conventionally be stopped after continuous use for one week or two to exchange a worn roll to new one. This drastically lowers the galvanizing efficiency and increases the plating cost.
JP2001-89836A discloses a roll for use in a galvanizing pot, which comprises a hollow roll body and shaft portions both made of a silicon nitride ceramic, the shaft portions being fit or screwed to both end portions of the roll body. Because this roll is entirely made of a ceramic, it has excellent corrosion resistance, heat resistance and wear resistance.
JP2001-89837A discloses a roll for continuous galvanization comprising a hollow roll body and shaft portions both made of a silicon nitride ceramic, the shaft portions being connected to both end portions of the roll body by fitting or screwing, and the shaft portions being provided with apertures on their peripheral surfaces for discharging a molten metal therethrough.
JP2003-306752A discloses a roll for continuous galvanization comprising a hollow roll body, shaft portions and a driving clutch portion each made of a ceramic, the shaft portions being fit to both end portions of the roll body such that there is a gap between the inner surface of the roll body and the outer surface of the shaft portion, the driving clutch portion being fit to a driving-side shaft portion such that there is a gap between the outer surface of the driving-side shaft portion and the inner surface of the driving clutch portion, and each fitting portion being fixed by bolts, pins, etc.
However, the silicon nitride ceramics used in JP2001-89836A, JP2001-89837A and JP2003-306752A are, for instance, sialon comprising 87% by weight of α-Si3N4, 5% by weight of Al2O3, 3% by weight of AlN, and 5% by weight of Y2O3, having thermal conductivity of at most about 17 W/(m·K), and insufficient heat shock resistance. Accordingly, when it is immersed in a molten metal pot, it is likely to be broken by heat shock.